The present invention relates to a process for making microporous structures for use as a catalyst support. In particular, the present invention relates to a process for making titanium dioxide suitable as a catalyst structure for fuel cells, sensors, electrochemical cells, and the like. The present invention also relates to a catalyst structure formed from the process of the present invention.
Different materials and processes are known for the manufacture of catalyst supports. High porosity and good physical strength are general requirements for such products. High temperature stability of the structure is also required if the catalyst operates at elevated temperature.
U.S. Pat. No. 5,036,037 teaches a method to produce metal oxide catalysts by pyrohydrolysis from solutions of chlorides, fluorides, or nitrates. The process makes particles with a mean size of 20-30 microns and with a high specific surface area. The processing temperature is at least 500xc2x0 C. and generally higher than 700xc2x0 C., to avoid the presence of the anion (chloride or fluoride) in the oxide product. The product can be used as such or can be further treated to give it the required physical or chemical properties. Although this process is suitable for the intended product, improvements to the process are desired.
Novel processes for the manufacture of titanium dioxide from aqueous solutions have been disclosed in PCT Publications WO 01/00530, WO 01/00531, and WO 01/12555, the relevant portions of which are incorporated herein by reference. In general, these applications describe the processing of an aqueous solution of a titanium salt by evaporation to produce an intermediate. The evaporation is conducted at a temperature higher than the boiling point of the solution, but lower than the temperature where significant crystal growth of an oxide phase occurs. In some embodiments, the evaporation may be conducted at a temperature higher than the boiling point of the solution but lower than the calcination temperature of the intermediate.
In the case of titanium solutions, the temperature generally ranges from 120xc2x0 to 350xc2x0 C., and preferably from 200xc2x0 to 250xc2x0 C. The process is preferably conducted by spraying, and can be accomplished in a spray dryer. The spray drying process produces thin-filmed spheres or parts of spheres, with a diameter of about 1 to 100 xcexcm, and a shell thickness of about 0.03 to 5 xcexcm.
After calcination and milling of these spheres or parts of spheres, and depending on the conditions of evaporation, the choice of additives, and the conditions of calcination, ultra-fine nano-sized TiO2 or, alternatively, pigment grade TiO2 can be obtained.
There has been no suggestion, however, that such a process can economically and commercially produce catalyst structures made of metal oxides from salt solutions of the metals. The present invention is therefore directed to a process to economically produce catalyst structures or catalyst supports.
Accordingly, the present invention teaches a process to produce catalysts or catalyst structures with high porosity, high specific surface area, high mechanical strength, and excellent thermal stability. In contrast to the method disclosed in U.S. Pat. No. 5,036,037, the method of the present invention uses lower temperature equipment for the first step of the process and adjunction of chemical control additives. The method uses a combination of spraying, pressing, and crystallization, which allows optimal control of the physical characteristics of the product.
The present invention provides a process for making catalysts or catalyst structures that comprises mixing an aqueous solution of a metal salt and a chemical control agent to form an intermediate solution. The solution is preferably free of any precipitate.
The intermediate solution is then evaporated to form an intermediate product. The evaporation is conducted under conditions to achieve substantially total evaporation. In particular, the evaporation is conducted at a temperature higher than the boiling point of the feed solution but lower than the temperature where significant crystal growth occurs. The evaporation may be conducted at a temperature higher than the boiling point of the solution but lower than the crystallization temperature of the intermediate. In a particularly preferred embodiment, the intermediate is an amorphous solid formed as a thin film and preferably is spherical or part of a sphere.
The term xe2x80x9csubstantially total evaporationxe2x80x9d or xe2x80x9csubstantially complete evaporationxe2x80x9d refers to evaporation such that the solid intermediate contains less than 15% free water, preferably less than 10% free water, and more preferably less than 1% free water. The term xe2x80x9cfree waterxe2x80x9d is understood and means water that is not chemically bound and can be removed by heating at a temperature below 150xc2x0 C. After substantially total evaporation or substantially complete evaporation, the intermediate product will have no visible moisture present.
The intermediate product is then mixed with a binder to form a mixture that is then dried. The drying can be performed in any suitable manner but is preferably air dried. The dried mixture is then pressed into a desired shape. Suitable desired shapes include, but are not limited to disks, full cylinders or hollow cylinders in the size range of a few mm to about 20 cm.
The shaped product may then be heated to a temperature of about 100xc2x0 C. to remove any remaining moisture or volatile compounds and prevent cracks during the heat treatment (crystallization) step.
The heat treated product is then crystallized by raising the temperature to a temperature between about 500xc2x0 C. to about 1300xc2x0 C. for a period of time from about 2 to about 24 h and then cooled to room temperature. The cooled product is then washed by immersing it in water or dilute acid, heated to boiling and maintained at the boiling point for a period of time from about 5 min to 2 h, to remove traces of any water-soluble phase that may still be present after the crystallization step.
The product formed as a result of the process of the present invention includes a catalyst structure characterized by a porosity in the range of about 30% to about 70% and a thermal stability such that less than 5% dimensional change occurs upon holding the structure at a temperature of about 1100xc2x0 C. in an oxidizing atmosphere for 8 hours. A preferred product is a titanium dioxide catalyst structure consisting of needle-shaped particles that are strongly bound together while exhibiting a high porosity.